The recent development of mobile devices requires a power source having higher output and capacity. As such a power source, a lithium secondary battery, which can be charged and discharged, has been generally used. However, the lithium secondary battery has several problems in exhibiting the performance of electronic equipment having a relatively large capacity sufficiently and for a long period of time. Specifically, when manufacturing a large-capacity lithium secondary battery, the manufacturing costs of the lithium secondary battery are very high due to the characteristics of materials constituting the lithium secondary battery, the safety of the lithium secondary battery is low, and it takes a long period of time to charge the lithium secondary battery.
For this reason, development on new power generation systems that satisfy the aforesaid requirements while overcoming the limits of the lithium secondary battery are being actively carried out. Among them is a fuel cell that is capable of providing power at high efficiency for a long period of time.
The fuel cell is a battery that generates electricity by changing fuels, such as hydrogen and methanol, into water through an electrochemical reaction. The fuel cell has been attracting considerable attention as an environmentally-friendly energy source that can overcome the problems of the lithium secondary battery. Representative examples of the fuel cell are hydrogen fuel cell using a gaseous fuel and a direct methanol fuel cell using a liquid fuel, on which much research is being carried out and some of which is in a commercialization stage.
The direct methanol fuel cell is a battery using the reaction between methanol and oxygen. Specifically, the methanol is decomposed at a fuel electrode of the direct methanol fuel cell to create electrons and protons. The protons pass through a polymer electrolyte membrane and react with the oxygen at an air electrode to create water.
A membrane electrode assembly (MEA), which is one of components constituting the fuel cell, is a part at which the electrochemical reaction between the methanol and the oxygen occurs. The membrane electrode assembly includes a fuel electrode, an air electrode, and a polymer electrolyte membrane. At the fuel electrode, electrons are created by an oxidation reaction of the methanol. At the air electrode, water is created by a reduction reaction of the oxygen (air). The fuel electrode and the air electrode are constructed in a structure in which the electrode catalyst layers are coated on the electrolyte membrane.
FIG. 1 is a sectional view typically illustrating such a membrane electrode assembly. As shown in FIG. 1, a membrane electrode assembly 10 is constructed in a structure in which a fuel electrode 30 and an air electrode 40 are attached to opposite major surfaces of a polymer electrolyte membrane 20, respectively. Also, the fuel electrode 30 and the air electrode 40 are constructed in a structure in which catalyst layers 34 and 44 are disposed between the polymer electrolyte membrane 20 and a gas diffusion layer 32 and between the polymer electrolyte membrane 20 and a gas diffusion layer 42, respectively.
A direct methanol fuel cell including the membrane electrode assembly 10 with the above-stated structure has a problem in that a catalyst washing phenomenon occurs due to the liquid-phase methanol introduced to the fuel cell 30, unlike a common solid polymer electrolyte fuel cell. Specifically, the catalyst of the catalyst layer 34 coated on the electrolyte membrane 20 is separated from the catalyst layer 34 due to the continuous flow of the methanol with the result that the oxidation reaction area of the methanol is reduced, and therefore, the performance of the membrane electrode assembly 10 is decreased. Especially when a direct methanol fuel cell system is operated (driven) for a long period of time, the catalyst washing phenomenon acts as a principal factor that lowers the performance of the fuel cell.
In this connection, the present invention proposes a membrane electrode assembly constructed in a structure in which an additional porous membrane is attached to the opposite surface of the catalyst layer facing the polymer electrolyte membrane. On the other hand, some of conventional arts disclose a technology for attaching an additional layer (an adhesive layer) to the opposite surface of the catalyst layer facing the polymer electrolyte membrane.
For example, Japanese Unexamined Patent Publication No. 2004-214045 discloses a technology for applying an adhesive layer including conductive carbon and a hydrogen ion conductive polymer electrolyte to a gas diffusion layer and welding the gas diffusion layer to a catalyst layer at high temperature and high pressure so as to securely attach the catalyst layer and the gas diffusion layer to each other in a hydrogen fuel cell. Consequently, this technology may be directly applied to a direct methanol fuel cell so as to restrain the catalyst washing phenomenon. However, it has been proved that the flow of a liquid component, such as methanol, in the direct methanol fuel cell is difficult due to the thermally welded adhesive layer, and therefore, the reaction efficiency is considerably lowered.
Also, Japanese Unexamined Patent Publication No. 2005-174765 discloses a technology for positioning an absorption material, such as carbon black, absorptive polymer, carbon aerogel, metal oxide, or absorptive fiber, between a catalyst layer and a gas diffusion layer in a fuel electrode so as to accomplish the operation of a hydrogen fuel cell at low temperature. Consequently, this technology may be directly applied to a direct methanol fuel cell so as to restrain the catalyst washing phenomenon. As the direct methanol fuel cell is a system in which a liquid component, such as methanol, mainly acts, however, the absorption material is impregnated with a large amount of the liquid component in the above-described assembly structure, and the liquid component further decreases a coupling force of the catalyst layer, whereby the catalyst washing phenomenon is rather accelerated.
Consequently, there is a high necessity of a technology for restraining the phenomenon in which the catalyst is washed due to the liquid component in a direct liquid type fuel cell, such as a direct methanol fuel cell, without the reduction in operating efficiency of the fuel cell.